3.1.0 release notes: add section about mandatory plugins

Change-Id: Ied27c13ba2ef344ad84dac3dd29136e8dd6c38b2
diff --git a/pages/site/releases/3.1.md b/pages/site/releases/3.1.md
index dc10fb8..208dde7 100644
--- a/pages/site/releases/3.1.md
+++ b/pages/site/releases/3.1.md
@@ -8,6 +8,16 @@
 
 ## Release Highlights
 
+* Mandatory plugins
+
+  It is possible to configure which plugins are mandatory.
+
+  Gerrit will fail to start if a mandatory plugin cannot be loaded.
+
+  Disabling and restarting of a mandatory plugin is rejected.
+
+  Reloading of a mandatory plugin is still possible.
+
 ## Important Notes
 
 ### Schema Changes